While it doesn't affect your answer to this problem (since you are looking for the value at a specific time greater than zero), your y(t) is not quite correct. What is y(t) for t=-1sec? According to your answer it is 45 when it should be 0. Again, the implication of working with the one-sided function is that your inputs and outputs are one sided. So you need to multiply your result by u(t).